fix LevelIter.operator+/-
Merge pull request #29 from torbjoernk/hotfix/leveliterator-op-plus-minus

possible bugfix in `Controller::LevelIter`
docu: doxygen-related fixes

explicitly use Doxygen commands
docu: docstrings for Ecapsulation
docu: docstrings for VectorEcapsulation
docu: enable JAVADOC_AUTO_BRIEF

and got rid of `@brief` and `@details` sections
docu: using MathJAX markers directly (not Doxygen's)